net: stmmac: dwmac-sun8i: Balance internal PHY resource references

While stmmac_pltfr_remove calls sun8i_dwmac_exit, the sun8i_dwmac_init
and sun8i_dwmac_exit functions are also called by the stmmac_platform
suspend/resume callbacks. They may be called many times during the
device's lifetime and should not release resources used by the driver.

Furthermore, there was no error handling in case registering the MDIO
mux failed during probe, and the EPHY clock was never released at all.

Fix all of these issues by moving the deinitialization code to a driver
removal callback. Also ensure the EPHY is powered down before removal.

Fixes: 634db83b ("net: stmmac: dwmac-sun8i: Handle integrated/external MDIOs")
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: David S. Miller <davem@davemloft.net>